In today's marketplace, many products are packaged in sealed containers using vacuum packing. Often, this is done because the product is oxygen-sensitive and will degrade upon exposure to oxygen. Common examples of oxygen-sensitive products that use a vacuum packed container include coffee and dairy products.
Vacuum packing works by reducing the gas present in the headspace of a container. In this way vacuum packing lessens the exposure of the product to oxygen, and in turn, reduces the extent to which the product oxidizes and degrades before reaching the consumer. Vacuum packaging of products that generate off-gas, such as fresh coffee, also provides a means of sealing the final package before the product off-gassing has completed without generating high pressures in the finished package. The additional volume of gas generated by the product can increase the internal pressure of the package back to near ambient, but without the initial vacuum state, the sealed package may deform or burst due to high internal pressure. One problem with vacuum packaging of semi-rigid containers is the exposure of the structure to high stresses that require an overall stronger container to resist damage and deformation. Also for vacuum packaging of flexible packages, e.g. bags, enough internal package volume may not be available to adequately compensate for the off-gassing of the product without significant bulging of the package. A final problem with standard vacuum packing techniques used today is that pressure changes external to the container may damage the integrity of the vacuum seal, and therefore, allow oxidation of the product to occur. Such changes in pressure are known as pressure differentials.
A positive pressure differential, also called overpressure, exists when the pressure inside a sealed container is higher than the pressure of the external atmosphere. There are many factors that are known to cause overpressure. For example, the off-gas produced by a product sealed within a container may cause overpressure. Similarly, subjecting a sealed container to an increase in elevation may cause overpressure because as the altitude increases and the external pressure drops, the internal pressure of the container is effectively increased. Finally, subjecting a sealed container to an increase in temperature causes the gas volume inside the container to expand, thus increasing the internal pressure of the container relative to the external atmospheric pressure. Such overpressure may cause the seal of the container to fail, thereby exposing the product therein to oxygen. As explained above, oxidation may degrade the product and negatively effect the quality of the product before it reaches the consumer. Moreover, with enough overpressure the container may actually explode, which would result in a total loss of the product.
Conversely, a negative pressure differential, also called a vacuum, exists when the internal pressure of the container is less than the external pressure of the environment. There are many factors known to cause a vacuum. For example, the adsorption, absorption or reaction of gases generated by the product within the container may cause a reduction in the internal pressure of the container. Similarly, subjecting a sealed container to a decrease in elevation increases the external pressure in relation to the internal pressure of the container thereby causing a negative pressure differential. Finally, subjecting the sealed container to a decrease in temperature causes the gas volume within the container to contract, thus decreasing the internal pressure of the container in relation to the external pressure. Such a negative pressure may cause the vacuum to fail, thus oxidizing, and possibly degrading, the product. Moreover, if the vacuum is strong enough, it may cause the container to implode.
An alternative means to vacuum packing is through the use of a one way valve incorporated in the otherwise sealed package structure. The use of such a valve allows for off-gassing of a product, such as fresh ground coffee, in an otherwise sealed package without the need for an imposed vacuum to prevent excessive pressure within the package. By means of a venting valve, the deficiencies of vacuum packing of semi-rigid containers as well a flexible packaging can also be avoided.
Another advantage of packaging with a one-way valve is that overpressure issues can be avoided since the valve will vent away positive pressure differentials as described above during the life of the packaging. For example, it is known that sealed plastic containers with large headspace volumes have a tendency to fail when exposed to high altitudes, such as when the product is being transported across the Rocky Mountains in the United States with elevations as high as 11,000 feet above sea level. As aforementioned, this happens because high altitudes cause the pressure external to the container to drop. As a result, the internal pressure on the container is greater than the external atmospheric pressure. Without a way for the internal pressure to be released, the container can expand and bulge, eventually causing the seal to fail, resulting in exposure of the product to oxygen, which as aforementioned, may be damaging to the product if it is oxygen-sensitive. The use of a one-way pressure relief valve can eliminate this type of high altitude failure since the excess differential pressure will be vented from the package. One complication of this venting at high altitude is found when a package that has been equilibrated at high altitude is transported to a lower altitude and thus exposed to a higher external pressure. For a one-way valve that prevents the back flow of air into the package, the sealed package will have a lower internal pressure than the external atmospheric pressure. The valve must be able to withstand this internal vacuum without failure to prevent the ingress of oxygen that may potentially damage the contents.
Previous attempts to solve the problem of vacuum failure in one way valves have had limited success. For example, it is known to place a longitudinal channel along the width of a laminate to allow the off-gasses produced by the product within the container to escape. Such valves are described by Schultz in EP 0 144 011 B2. However, the disadvantage of this longitudinal channel is that air from the surrounding atmosphere can enter the container, thereby oxidizing and degrading the product. The unlaminated channel extending to a weld line, rim or edge of the container is extremely susceptible to air entering back through the valve structure due to the large stresses imparted by the imposed vacuum. Moreover, because the longitudinal channel extends the entire width of the laminate, there is an increased likelihood of vacuum failure due to the fact that the channel prevents proper sealing of the container.
Other attempts to provide valve structures suitable for use with a laminate include a liquid sealant in combination with an intermediate valve strip. Such valves are disclosed by Domke in U.S. Pat. No. 5,263,777. However, this approach requires the use of an additional intermediate layer of laminate, which increases the cost of manufacturing and raises the price of the final product.
Therefore, there is a need to provide cost efficient, flexible laminates having an integrated pressure release valve, which may be used in conjunction with a variety of containers for packing products under pressure.